DC electric arc furnaces are shown to the prior art as having an arcing electrode, of the graphite type, an electric connection for a melt in the furnace which does not become fully effective in the absence of a melt, and a withdrawable starting electrode for contact with metal pieces initially charged in the furnace, such as steel scrap.
In operation, the starting electrode is connected with, for example, the positive side of a power source with the arcing electrode connected with the negative side, the starting electrode being pressed into the scrap, the latter electrode normally being positioned so it can be moved up and down. In this way, the solid pieces are melted to form a melt in the furnace which, with time, comes into effective contact with the electric connection for such a melt and which is normally in the bottom of the furnace's hearth. After the melt is thus formed to that degree, the circuit including the starting electrode is switched off, and the circuit to the melt connection is switched on so that the arcing electrode can then carry out the metallurgical operation involved. When it is no longer needed, the starting electrode is withdrawn partially or completely from the furnace because it often is a metal electrode unable to withstand the heat of continued metallurgical operation.
The above has a substantial disadvantage because the high power of the electric currents required during the initial melt-down phase by the starting electrode's operation, and thereafter for powering through the hearth connection which continues in operation, require switches of very high current-carrying capacity even in the case of relatively small furnaces. In the case of furnaces having larger capacities than are usual for the described type of furnace, it becomes substantially impossible, either technically or economically, to provide switches of adequate current carrying capacity.
The object of the present invention is to provide for the operation of a furnace of the above-described general type, when because of larger capacities, for example, higher powered currents are involved than have heretofore been considered practicable.